The present invention relates to an apparatus for continuously separating polymer solids from a liquid medium comprising an inert diluent and unreacted monomers in a slurry polymerization process. In particular, the present invention relates to an apparatus for continuously separating polymer solids from a liquid medium, drying the polymer, and recovering the diluent and unreacted monomers with a reduction in compression needed for diluent vapor condensation to liquid diluent for reuse in the polymerization process. In another aspect, the invention relates to a method for continuously separating polymer solids from a liquid medium. In particular, the invention relates to a method for continuously separating polymer solids from a liquid medium, drying the polymer, and recovering the inert diluent and unreacted monomers for reuse in the polymerization process.
In many polymerization processes for the production of polymer, a polymerization effluent is formed which is a slurry of particulate polymer solids suspended in a liquid medium, ordinarily the reaction diluent and unreacted monomers. A typical example of such processes is disclosed in Hogan and Bank""s U.S. Pat. No. 2,285,721, the disclosure of which is incorporated herein by reference. While the polymerization processes described in the Hogan document employs a catalyst comprising chromium oxide and a support, the present invention is applicable to any process producing an effluent comprising a slurry of particulate polymer solids suspended in a liquid medium comprising a diluent and unreacted monomer. Such reaction processes include those which have come to be known in the art as particle form polymerizations.
In most commercial scale operations, it is desirable to separate the polymer and the liquid medium comprising an inert diluent and unreacted monomers in such a manner that the liquid medium is not exposed to contamination so that the liquid medium can be recycled to the polymerization zone with minimal if any purification. A particularly favored technique that has been used heretofore is that disclosed in the Scoggin et al, U.S. Pat. No. 3,152,872, more particularly the embodiment illustrated in conjunction with FIG. 2 of that patent. In such processes the reaction diluent, dissolved monomers, and catalyst are circulated in a loop reactor wherein the pressure of the polymerization reaction is about 100 to 700 psia. The produced solid polymer is also circulated in the reactor. A slurry of polymer and the liquid medium is collected in one or more settling legs of the slurry loop reactor from which the slurry is periodically discharged to a flash chamber wherein the mixture is flashed to a low pressure such as about 20 psia. While the flashing results in substantially complete removal of the liquid medium from the polymer, it is necessary to recompress the vaporized polymerization diluent (i.e., isobutane) in order to condense the recovered diluent to a liquid form suitable for recycling as liquid diluent to the polymerization zone. The cost of compression equipment and the utilities required for its operation often amounts to a significant portion of the expense involved in producing polymer.
Some polymerization processes distill the liquefied diluent prior to recycling to the reactor. The purpose of distillation is removal of monomers and light-end contaminants. The distilled liquid diluent is then passed through a treater bed to remove catalyst poisons and then on to the reactor. The equipment and utilities costs for distillation and treatment can be a significant portion of the cost of producing the polymer.
In a commercial scale operation, it is desirable to liquefy the diluent vapors at minimum cost. One such technique used heretofore is disclosed in Hanson and Sherk""s U.S. Pat. No. 4,424,341 in which an intermediate pressure flash step removes a significant portion of the diluent at such a temperature and at such a pressure that this flashed portion of diluent may be liquified by heat exchange instead of by a more costly compression procedure.
The present invention relates to an apparatus for continuously separating polymer solids from a liquid medium comprising an inert diluent and unreacted monomers. In another aspect, the invention relates to an apparatus for continuously separating polymer solids from a liquid medium, drying the polymer, and recovering the diluent and unreacted monomers with a reduction in compression needed for diluent vapor condensation to liquid diluent for reuse in a polymerization process. In another aspect, the invention relates to a method for continuously separating polymer solids from a liquid medium. In another aspect, the invention relates to a method for continuously separating polymer solids from a liquid medium, drying the polymer, and recovering the inert diluent and unreacted monomers for reuse in a polymerization process.
In accordance with the present invention, there is provided an apparatus for continuously recovering polymer solids from a polymerization effluent comprising a slurry of said polymer solids in a liquid medium comprising an inert diluent and unreacted monomers. The apparatus comprises a discharge valve on a slurry reactor, examples of which include slurry loop reactors and stirred tank slurry reactors, for the continuous discharge of a portion of the slurry reactor contents into a first transfer conduit: a first flash tank having a bottom defined by substantially straight sides inclined at an angle to the horizontal equal to or greater than the angle of slide of the slurry/polymer solids; wherein the pressure of the first flash tank and temperature of the polymerization effluent are such that from about 50% to about 100% of the liquid medium will be vaporized and the inert diluent component of said vapor is condensable, without compression, by heat exchange with a fluid having a temperature in the range of about 65xc2x0 F. to about 135xc2x0 F.: a first flash tank exit seal chamber, communicating with said first flash tank, of such a length (l) and diameter (d) as to permit such a level of concentrated polymer solids/slurry to accumulate and form a pressure seal in said first flash tank exit seal chamber: a seal chamber exit reducer providing for a continuous discharge of a plug flow of concentrated polymer solids/slurry to a second transfer conduit which communicates the concentrated polymer solids/slurry into a second flash tank wherein the pressure of said second flash tank and temperature of the concentrated polymer solids/slurry are such that essentially all of any remaining inert diluent and/or unreacted monomer will be vaporized and removed overhead for condensation by compression and heat exchange and the polymer solids are discharged from the bottom of said second flash tank for additional processing or storage.
The invention provides also a method for the continuous removal of a stream of polymerization effluent from a slurry reactor through a discharge valve; increasing the heat content of the polymerization effluent during its transit through said first transfer conduit to a temperature below the fusion point of the polymer while continuously communicating the polymerization effluent to a first flash tank having a bottom defined by substantially straight sides inclined at an angle to the horizontal equal to or greater than the angle of slide of the concentrated polymer solids/slurry; continuously vaporizing from about 50% to about 100% of the liquid medium in said first heated flash tank to yield a concentrated polymer solids/slurry and a vapor stream at such a temperature and pressure that the inert diluent content of said vapor is condensable, without compression, by heat exchange with a fluid having a temperature in the range from about 65xc2x0 F. to about 135xc2x0 F.; continuously discharging the concentrated polymer solids/slurry from said first flash tank to a first flash tank exit seal chamber of such a length (I) and diameter (d) that a volume of concentrated polymer solids/slurry is continuously maintained so as to form a pressure seal in said first flash tank exit seal chamber; continuously discharging the concentrated polymer solids/slurry from said first flash tank seal chamber through a seal chamber exit reducer defined by substantially straight sides inclined at an angle to that of horizontal equal to or greater than the angle of slide of the polymer solids which remain after removal of about 50 to 100% of the inert diluent therefrom; communicating a continuous plug flow of concentrated polymer solids/slurry from said first flash tank exit seal chamber through said seal chamber exit reducer to a second transfer conduit which communicates said continuous plug flow of concentrated polymer solids/slurry to a second flash tank; and continuously vaporizing essentially all of any remaining inert diluent and/or unreacted monomer in a second flash tank operated at a lower pressure than said first flash tank; condensing the vaporized inert diluent and/or unreacted monomer from said second flash tank by compression and heat exchange; and continuously discharging the essentially dried polymer slurry from said second flash tank for further processing or storage.
The present invention also relates to an apparatus for capturing a higher weight percentage of polymer solids from a circulating slurry in a loop reactor than the weight percentage of solids in the circulating slurry. The apparatus includes a conduit having a first end, wherein said first end extends for a distance into the loop reactor. The conduit also has portions defining an opening wherein said opening is positioned relative to the direction of the circulating slurry. Desirably, the opening may be facing the direction of flow of the circulating slurry. Additionally, a portion of the conduit may extend outwardly from the loop reactor for discharging, continuously or otherwise the polymer solids from the loop reactor.
The present invention also provides a process for capturing a higher weight percentage of polymer solids from a circulating slurry in a loop reactor than the weight percentage of polymer solids in the circulating slurry. This process includes the step of extending for a distance into a the loop reactor a conduit having portions defining an opening wherein said opening is extends into the circulating slurry. Additionally, this process may include the step of discharging, continuous or otherwise, the polymer solids from the loop reactor through a portion of the conduit extending outwardly from the loop reactor.
The present invention also provides an apparatus for purging polymer solids from a conduit connected to a loop reactor and in fluid communication with the loop reactor. This apparatus includes a sensor, a first valve in fluid communication with the conduit, a second valve positioned between a first inert diluent and the conduit, wherein the first inert diluent is in fluid communication with the conduit between the loop reactor and the first valve. In response to a signal produced by the sensor, the first valve is closed and the second valve is opened allowing the first inert diluent to enter the conduit in sufficient quantities and under sufficient pressure to purge polymer solids from the conduit. This apparatus may further include a third valve positioned between a second inert diluent and the conduit, wherein the second inert diluent is in fluid communication with the conduit between the loop reactor and the first valve. In this way, when the first valve is open and the second valve is closed the third valve is opened allowing the second inert diluent to enter the conduit.
The present invention also provides a process for purging polymer solids from a conduit connected to a loop reactor and in fluid communication with the loop reactor comprising. This process includes the steps of (i) closing a first valve in response to a first signal from a first sensor, wherein the first valve is connected to and in fluid communication with the conduit, (2) opening a second valve in response to a second signal from a second sensor, wherein the second valve is fluid communication between a first inert diluent and the conduit, and wherein the first inert diluent is in fluid communication with the conduit between the loop reactor and the first valve, and (3) flowing sufficient quantities of the first inert diluent under sufficient pressure into the conduit to purge polymer solids from the conduit. In this process the first and second sensors may be a common sensor and the first and second signal may be a common signal.
The present invention also provides an apparatus for returning fines to a polymerization slurry in a loop reactor. The apparatus includes a discharge valve for discharging a portion of the polymerization slurry from the loop reactor into a first transfer conduit. The first transfer conduit communicates the polymerization slurry into a first flash tank. The first flash tank converts a portion of the polymerization slurry into a first fluid, such as a vapor. The first fluid includes a portion of the diluent and the fines from the polymerization slurry. A second transfer conduit communicates the first fluid to a first cyclone. The first cyclone converts a portion of the first fluid into a second fluid, such as a vapor. The second fluid includes a portion of the diluent and the fines. A third transfer conduit communicates the second fluid into a heat exchanger. The heat exchanger converts the second fluid into a liquid comprising the diluent and the fines. A fourth transfer conduit returns the liquid to the polymerization slurry in the loop reactor. This apparatus may also include a first transfer conduit heater for heat exchange between the first transfer conduit heater and the polymerization slurry.
The present invention also provides a process for returning fines to a polymerization slurry in a loop reactor. The process includes (i) discharging a portion of the polymerization slurry from the loop reactor, (ii) communicating the discharge polymerization slurry into a first flash tank, (iii) converting in the flash tank a portion of the polymerization slurry into a first fluid, the first fluid comprising a diluent and the fines, (iv) communicating the first fluid from the first flash tank to a first cyclone, (v) converting in the cyclone a portion of the first fluid into a second fluid comprising the diluent and the fines, (vi) communicating the second fluid into a heat exchanger, (vii) converting in the heat exchanger the second fluid into a liquid comprising the diluent and the fines, and (viii) returning the liquid to the polymerization slurry in the loop reactor.
The present invention further provides an apparatus and process for producing polymer from a polymerization slurry in a loop reactor operating at a space time yield greater than 2.8 lbs/hr-gal. In this instance, the polymer is formed in the polymerization slurry which includes a liquid medium and solids. The polymerization slurry is discharged into a first transfer conduit. The polymerization slurry is referred to as a polymerization effluent upon leaving the loop reactor. The polymerization effluent is heated in the first transfer conduit to a temperature below the fusion temperature of the polymer solids. The heated polymerization effluent is communicated through said first transfer conduit to a first flash tank. In the first flash tank, from about 50% to about 100% of the liquid medium is vaporized. The vapor is condensed by heat exchange. Polymer solids are discharge from the first flash tank to a second flash tank through a seal chamber of sufficient dimension such as to maintain a volume of polymer solids in the said seal chamber sufficient to maintain a pressure seal. The polymer solids are then communicated to a second flash tank. In the second flash tank, the polymer solids are exposed to a pressure reduction from a higher pressure in the first flash tank to a lower pressure in said second flash. The polymer solids are then discharging from said second flash tank. Additionally, the weight percent of solids in the polymerization slurry may be greater than 47. The loop reactor may be operated at a total recirculating pumping head/reactor distance of greater than 0.15 ft/ft. The loop reactor may also be operated with a recirculating pumping head greater than or equal to 200 ft. and have more than eight vertical legs, desirably between 10 and 16 vertical legs, more desirably between 10 and 12 vertical legs, most desirably 12 vertical legs. The volume of polymerization slurry in the loop reactor may be greater than 20,000 gallon.
An object of the present invention is to provide both an apparatus and method for the continuous two stage flash drying of the polymer solids following the continuous removal of the polymerization effluent comprising polymer solids and liquid medium comprising inert diluent and unreacted monomers from a slurry reactor through a point discharge valve, a continuous solids level control in the first flash tank exit seal chamber that provides a pressure seal therein which enables said first flash tank to operate under a substantially greater pressure than said second flash tank while polymer solids are continuously discharged through the seal chamber exit reducer into the second transfer conduit and further into the second flash tank which eliminates plugging in the first flash tank and the continuous liquification of from about 50% to about 100% of the inert diluent vapor by heat exchange rather than compression.
Another object of the invention is to eliminate the need for a settling leg on the slurry reactor and the intermittent high pressure pulse in the slurry reactor caused by periodic discharging of the contents of the settling leg. Another object of the present invention is to improve safety by eliminating the possibility of plugging in a settling leg.
Another object of the invention is to eliminate plugging in equipment downstream from the discharge valve. In a settling leg of a polymerization reactor polymerization continues and the heat of reaction further heats the liquid medium and a potential exists for some of the polymer solids to dissolve or to fuse together. As the contents of the settling leg exit the discharge valve, the pressure drop causes flashing of some of the liquid medium which results in cooling the remaining liquid medium causing the dissolved polymer to precipitate which tends to plug downstream equipment. The present invention which eliminates the need for a settling leg also eliminates this potential for downstream equipment plugging by avoiding the initial dissolution or fusion of the polymer solids.
Another object of the present invention is to increase the reactor through-put by the use of continuous discharge and increased ethylene concentrations in the liquid medium, e.g., greater than or equal to 4 weight percent at reactor outlet, desirably from 4 weight percent to 8 weight percent, still more desirably from 5 weight percent to 7 weight percent. Settling legs limit ethylene concentrations due to an increased tendency to plug downstream equipment caused by accelerated reaction within the settling leg. A continuous polymerization effluent slurry flow allows ethylene concentrations to be limited only by the ethylene solubility in the liquid diluent in the reactor, thereby increasing the specific reaction rate for polymerization and increasing reactor throughput.
Another object of the present invention is to increase the weight percent (wt %) of polymer solids in the polymerization slurry circulating in the polymerization zone in the loop reactor. Desirably, the wt % of polymer solids in the polymerization slurry is greater than 45, more desirably, from 45 to 65, still more desirably from 50 to 65, and most desirably from 55 to 65.
Another object of the present invention is to increase the space time yield (STY), expressed in terms of pounds per hour-gallon (lbs/hr-gal). Desirably, the STY is greater than 2.6, more desirably from 2.6 to 4.0, and most desirably from 3.3 to 4.0.
Other aspects, objects, and advantages of the present invention will be apparent from the following disclosure and FIGS. 1 and 2.
The claimed apparatus and process provide several advantages over the prior art including: (1) allowing for a continuous processing of the contents of a slurry reactor from the point of discharge of the polymerization slurry effluent through a discharge valve; a first flash tank; a seal chamber; a seal chamber exit reducer; and therefrom to a second flash tank, (2) significantly increasing ethylene concentration in the loop reactor liquid medium thereby increasing reactor through-put, (3) significantly increasing the wt % of polymer solids in the polymerization slurry, (4) significantly increasing reactor space time yield and (5) energy consumption is reduced by reducing the need to compress and/or distill the reactor vapor-liquid effluent. Recycling compressors and other downstream equipment can be reduced in size or eliminated.